Electronic displays are provided in many contexts to electronically render digital information to a viewer. The electronic displays receive information, and render the information through lighted cells in patterns that reflect the texts and pictures employed to convey the information.
A heads-up display (HUD) allows a viewer to view not only the lighted information, but due to the transparent nature of the HUD, the view through the HUD. Thus, a viewer may be delivered information while not losing the ability to view the real world through the HUD.
As shown in FIG. 1, a HUD 100 may include at least a projection unit 110 (or picture generating source), a combiner 120, and a video generation computer 130. Although shown as separate components, all the elements together compose a singular HUD implementation.
The projection unit 110 in a HUD 100 has a convex lens or concave mirror with a light emitting technology (for example, a Cathode Ray Tube (CRT), light emitting diode, or liquid crystal display at its focus). This setup produces an image where the light is collimated, i.e. the focal point is perceived to be in front of the windscreen at a predetermined distance.
The combiner 120 is typically an angled flat or curved piece of glass (a beam splitter) located directly in front of the viewer, that redirects the projected image from a projection unit 110 in such a way as to see the field of view and the projected image at the same time. Combiners 120 may have special coatings that reflect the light projected onto it from the projector unit 110 while allowing all other wavelengths of light to pass through. In some optical layouts, combiners may also have a curved surface to refocus the image from the projecting unit 110.
The computer 130 provides the interface between the HUD 100 and the systems/data to be displayed and generates the imagery to be displayed by the projection unit 110.
In certain HUD applications, the illumination may be provided with additional properties to improve the image. One such technique employed in HUD applications is the use of s-polarization (i.e. an s-polarized optical wave). S-polarization adds optical power to the content being displayed because of a higher reflectivity associated with the s-polarization.
The main reason to employ s-polarization in a HUD implementation is that in some designs there is no coating on the combiner or polarization dependent structure in the media such as grating. Thus, the reflectivity of s-polarization is always greater than the reflectivity of p-polarization.
However, many viewers employ eyewear or other viewing devices that use p-polarization (for example, sunglasses). These devices are employed to avoid deleterious effects caused by environmental conditions, such as sun light.
Thus, by employing an s-polarization component with eyewear that employs p-polarization, the viewer of the HUD is left with a degraded or non-visible image. Several techniques may be employed to address this, such as providing a quarter-wave plate. However, this solution requires additional power to maintain the brightness when a viewer is not wearing a polarized eyewear device.